# Homework Help: Periodic or not? Determine the wave function

1. May 20, 2013

### jackscholar

1. The problem statement, all variables and given/known data
The following picture is supposedly periodic (or at least my teacher says so). Could anybody suggest where I begin in order to determine the wave function for this messy graph. Please see the attached for the graph.

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2. May 20, 2013

### Simon Bridge

What is the graph supposed to represent? What's the context?
What level are you doing this at?

It could be noisy data.

3. May 20, 2013

### jackscholar

Very basic, nothing like fourier analysis. Only sine and cosine waves unfortunately. It is more of a "try and recognise the periodic nature of the wave" task. The purpose of it is for biorhythms, unfortunately I have to find periodic motion within the graph otherwise I get a lower grade.

4. May 20, 2013

### Simon Bridge

So it's just a bunch of made-up numbers?
The scales on the axis don't mean anything?
It's not, say, the time for a particular number of heartbeats taken each morning?
Context is everything.

Certainly looks nothing like a sine wave unless it's very noisy

Biorhythms? The attempt to predict various aspects of a person's life through simple mathematical cycles?
That would be pseudoscience.

Proponents are always finding cycles in noise... you start out by assuming it's there.
i.e. squint and blur your eyes and stand on your head and smoke something illegal you see it has a trough at about day 25 and another one at about day 145. Like I said: not science.

5. May 20, 2013

### jackscholar

I understand where you're coming from but it is slightly different to what you're thinking. I was given sample data for an assignment. I by no means subscribe to biorhythms but I have to pretend I do so I can complete this assignment. The x-axis represents the days in increments of 4and the y-axis represents the time taken to complete a task. Supposedly there is meant to be a relationship between the days and the time taken such that it forms a wave. Regardless of whether or not I believe in biorhythms I have to find one [wave function] present within the graph given so I can pass.

6. May 20, 2013

### Simon Bridge

Cool - so we are trying to fit a sine wave to noisy data.
Did you learn about curve fitting in class at all?

It could be a problem ... where you are supposed to disprove the authoritatively stated hypothesis - like Marie Curie and the goldfish?

Anyway - like I said - you can see parts of the graph where the data tends to favor low values and parts where it tends to favor high ones. Those would correspond to troughs and peaks of a sine wave - which tells you the period. I take it figuring the amplitude is no problem? That leaves the phase.